The present invention relates to a synchronous electric motor control apparatus employed in a system for driving a synchronous electric motor by using a discharge output of a battery and used for controlling a synchronous electric motor in accordance with a torque command.
In a system for driving a synchronous electric motor by using a discharge output of a battery, it is quite within the bounds of possibility that the voltage of the electric motor exceeds the voltage of the battery at a high-speed region. In order to solve this problem, field weakening control to forcibly weaken the field magnetic flux of the electric motor is adopted in a variety of motor driving systems. To put it concretely, the field weakening control reduces the intensity of a field magnetic flux generated in a winding or a magnet by controlling a command of a d-axis current which is a part of an electric motor current contributing to generation of the field magnetic flux. Depending on factors such as the performance of the battery, a load condition and a residual electric charge, however, it is still quite within the bounds of possibility that the voltage of the electric motor exceeds the voltage of the battery, damaging the control apparatus even if the field weakening control is executed.
In order to solve the above problem, there has been introduced a technique of executing field weakening control after waiting for variations in battery voltage caused by a load of the electric motor to get stabilized as is disclosed in Japanese Patent Laid-open No. Hei 7-246000.
Japanese Patent Laid-open No. Hei 7-107772 discloses a technique of field weakening control which, for a rotational speed of an electric motor exceeding a base rotational speed, comprises the steps of computing a maximum voltage that can be output by the electric motor from the voltage of a battery; computing a weakening field current command Id* from the computed maximum voltage of the electric motor and executing the field weakening control based on the computed current Id*. Japanese Patent Laid-open No. Hei 10-136699 discloses a technique of correction and reduction of a torque command given to the electric motor so as to merely make an inferred value of the electric motor voltage below a value corresponding to a detected value of the battery voltage.
The conventional technology is effective for execution of field weakening control with the voltage of the electric motor never exceeding the voltage of the battery due to correction of a torque command. When it is desired to output a desired torque at a desired rotational speed, however, depending on the voltage of the battery, a sufficiently large torque may not be obtained for some cases. In addition, since the base rotational speed changes when the voltage of the battery changes, optimum field weakening control can not be executed in some cases due to the variable base rotational speed.